Semiconductor manufacturers compete in a highly competitive and capital-intensive industry. A state-of-the-art semiconductor fabrication facility can cost $1 billion or more and include hundreds of different fabrication tools including, for example, material stockers, chemical vapor deposition (CVD) systems, photolithography systems, chemical-mechanical planarization systems, etc. On any given day, a number of different batches of semiconductor wafers can run through a fabrication facility. To recover the capitalization costs and remain competitive, semiconductor manufacturers continually seek to improve semiconductor throughput and yield.
Manufacturers pay particular attention to the photolithography process, a process which occupies a substantial amount of floor space and is performed on a given wafer a number of different times throughout the fabrication process. The photolithography process typically involves applying a photoresist layer (e.g., SiO.sub.2) over the surface of a semiconductor wafer using a coating machine or coater. The wafer then moves to an exposure tool, such as a photolithography stepper, which exposes the photoresist layer to a patterned light source. The light source is typically patterned using a mask or reticle (hereinafter reticle). The reticle typically contains clear and opaque features which generally define the pattern to be created in the photoresist layer. The exposed photoresist is then developed and regions of the photoresist are dissolved such that the pattern is transferred to the photoresist layer. The exposed regions of the underlying semiconductor wafer layer are then processed by, for example, etching the exposed wafer layer, depositing a material on the exposed wafer layer, or doping the exposed wafer layer.
For a given wafer, the photolithography process may be used a number of times as layers are formed over layers to form the ultimate semiconductor device structure. To perform the various photolithography processes, a typical plant employs upwards of 10-20 different photolithography steppers and can use thousands of different reticles. Given the substantial use of photolithography in the semiconductor fabrication process, manufacturers find any increase in the throughput of photolithography systems to be highly desirable.